The invention relates to clutches in general, and more particularly to improvements in friction clutches. Still more particularly, the invention relates to improvements in friction clutches which can be utilized with advantage in power trains between the engines and the wheels of motor vehicles.
A friction clutch of the type to which the present invention pertains normally employs a housing or cover which receives torque from the prime mover of a vehicle, for example, by way of a counterpressure plate (such as a flywheel) which is driven by the output element (e.g., a camshaft or a crankshaft) of a combustion engine. An axially movable pressure plate is installed between the housing and the counterpressure plate and is caused to bias a clutch disc against the counterpressure plate when the clutch is engaged. The clutch disc then transmits torque to the input shaft of a transmission or another torque receiving part in the power train of the vehicle. The means for biasing the pressure plate toward the counterpressure plate (to thus clamp the clutch disc between the two plates in the engaged condition of the clutch) normally comprises a clutch spring (particularly a diaphragm spring) which reacts against the housing and bears against the adjacent side of the pressure plate when the clutch is engaged. The pressure plate shares the angular movements of the housing and of the counterpressure plate; to this end, the pressure plate is normally coupled to the housing by leaf springs which permit the pressure plate to move toward and away from the counterpressure plate in the direction of the common axis of the housing and the counterpressure plate but prevents the pressure plate from turning relative to the housing.
It is further known to provide a friction clutch of the above outlined character with a unit which can compensate (preferably automatically) at least for wear upon friction linings which form part of the clutch disc and are in frictional contact with the pressure plate and the counterpressure plate in the at least partially engaged condition of the clutch.
Friction clutches of the aforedescribed type are described and shown, for example, in published German patent application Serial No. 42 39 289 as well as in the prior art which is referred to in such application.
An advantage of the wear compensating unit is that it renders it possible to bias the pressure plate and the counterpressure plate of the friction clutch against the adjacent friction linings of the clutch disc with a considerable force even after the friction linings have already undergone extensive wear. Another purpose of the wear compensating unit is to render it more likely that the clutch can be disengaged in response to the application of a relatively small disengaging force, namely a disengaging force whose maximum value can be said to be within an acceptable (relatively low) range, as well as to establish circumstances under which the disengaging force is likely to remain at least substantially constant during the useful life of the clutch. The useful life of the clutch is determined, to a considerable degree, by the useful life of the friction linings on the clutch disc even though certain other factors (such as the wear upon the pressure plate, the wear upon the counterpressure plate and/or the wear upon the clutch spring or springs) are also apt to affect the life expectancy of the friction clutch.
The utilization of diaphragm springs as a means for biasing the pressure plate against the adjacent friction linings of the clutch disc in the engaged condition of the friction clutch has been found to exhibit the advantage that the clutch can be disengaged in response to the application of a relatively small (maximum) disengaging force and that the disengaging force need not fluctuate within a relatively wide range. The nature and the mounting of the diaphragm spring (which serves to bias the pressure plate against the clutch disc in the engaged condition of the friction clutch) are such that the magnitude of the force being applied by the diaphragm spring decreases at least during the major part of the distance that must be covered by the pressure plate for the purpose of disengagement of the clutch, i.e., the diaphragm spring exhibits a degressive force-to-distance characteristic.
However, the aforedescribed conventional friction clutches also exhibit certain serious drawbacks. Thus, even if the means for engaging the clutch employs one or more diaphragm springs, the magnitude of the disengaging force which must be applied in order to move the pressure plate away from the clutch disc often varies within an excessively wide range. As a rule, the disengaging force which must be applied to move the pressure plate away from the counterpressure plate at first rises toward an undesirably high maximum value to thereupon gradually decrease to a minimum value. The difference between the maximum and minimum values of such disengaging force is considerable and, in many instances, well above the acceptable range. As a rule, the ratio of the maxima and minima of disengaging forces which must be applied in heretofore known friction clutches of the above outlined character is between 1.5 and 2.5 or even higher.
The need for the application of very pronounced disengaging forces is undesirable irrespective of whether the clutch is being manipulated by the operator of the vehicle or automatically, e.g., by a servomotor. If the clutch is to be disengaged by a motor, the dimensions of such motor are proportional to the magnitude of the disengaging force which is to be applied in order to move the pressure plate away from the counterpressure plate. The same applies for the energy requirements of the motor. Moreover, it is necessary to employ stable and hence bulky and more expensive means for transmitting motion in order to disengage the clutch. Rather bulky, heavy and expensive motion transmitting means are necessary on the additional ground that such parts must withstand pronounced deforming stresses during transmission of large forces from the disengaging motor to the pressure plate in order to disengage the clutch. Even the elasticity of the motion transmitting parts must be limited to a minimum in order to ensure predictable disengagement of the clutch against the opposition of the diaphragm spring(s).